Since an aerogel, as a high specific area (≥500 m2/g), ultra-porous material having a porosity of about 90% to about 99.9% and a pore diameter of about 1 nm to about 100 nm, has excellent characteristics such as ultra lightweightness, ultra insulation, and ultra-low dielectric constant, research into the applications of the aerogel as a transparent insulator and an environmentally-friendly high-temperature insulator, an ultra-low dielectric thin film for a highly integrated device, a catalyst and a catalyst support, an electrode for a supercapacitor, and an electrode material for desalination as well as the development of an aerogel material has been actively conducted.
The biggest advantage of the aerogel is super-insulation having a thermal conductivity of 0.300 W/m·K or less which is lower than that of an organic insulation material such as a typical Styrofoam. Also, the aerogel may address fire vulnerability and generation of toxic gas in case of fire, i.e., fatal weaknesses of a typical organic insulation material.
In general, a wet gel is prepared from a silica precursor such as water glass or tetraethoxysilane (TEOS), and an aerogel is then prepared by removing a liquid component in the wet gel without destroying its microstructure. A silica aerogel may be categorized into three typical forms, powder, granules, and monolith, and the silica aerogel is generally prepared in the form of powder.
The silica aerogel powder may be commercialized in a form, such as an aerogel blanket or aerogel sheet, by compositing with fibers, and, since the blanket or sheet has flexibility, it may be bent, folded, or cut to a predetermined size or shape. Thus, the silica aerogel may be used in household goods, such as jackets or shoes, as well as industrial applications such as an insulation panel of a liquefied natural gas (LNG) carrier, an industrial insulation material and a space suit, transportation and vehicles, and an insulation material for power generation. Furthermore, in a case where a silica aerogel is used in a fire door as well as a roof or floor in a home such as an apartment, it has a significant effect in preventing fire.
However, since the silica aerogel powder may be scattered due to high porosity, very low tap density, and small particle size, handling may be difficult and fill may not be easy.
Also, although the silica aerogel monolith has high transparency in visible light region, the silica aerogel monolith may have a size limitation, may be difficult to be molded in various shapes, and may be easily broken.
In order to address the above-described limitations of the silica aerogel powder and monolith, attempts have been made to increase ease of handling and shape-responsiveness by preparing silica aerogel granules having a diameter of 0.5 mm or more. For example, there are methods such as the method in which a reaction solution obtained by hydrolyzing alkoxysilane is prepared as a filler, gelation is performed by polycondensation of the filler with a catalyst, a hydrophobic treatment is performed by reacting with a hydrophobic agent, and supercritical drying is then performed to obtain hydrophobic silica aerogel granules; and the method in which aerogel particles including additives and binder are supplied to a molding machine and compressed to prepare silica aerogel granules.
However, since the above-described methods use an ancillary granulating device and an additive such as a binder, technically complex process and long process time may not only be required, but complex processing procedures and high investment costs may also be required when a silica aerogel is mass-produced by the above-described methods. As a result, a lot of time and expensive chemicals are required, and accordingly, production costs may not only be increased, but also a particle size of the finally obtainable silica aerogel may not be uniform or may be excessively large.
Furthermore, since gel structure characteristics and physical properties are reduced when the silica aerogel absorbs moisture, there is a need to develop a method, which may permanently prevent the absorption of moisture in the air, for ease of use in industry. Thus, methods of preparing a silica aerogel having permanent hydrophobicity by performing a hydrophobic treatment on a surface of the silica aerogel have been proposed. In general, a silica aerogel having hydrophobicity is being prepared by using a surface modifier. However, in a case in which the surface of the silica aerogel is hydrophobized only with the surface modifier, since a large amount of the expensive surface modifier may be used and it may be difficult to control a surface modification reaction, productivity and economic efficiency are low and there are limitations in preparing a silica aerogel having high hydrophobicity.
Therefore, there is a need to develop a method which may prepare a silica aerogel having high hydrophobicity by controlling hydrophobicity while easily controlling a surface modification reaction.